Starting from AMPS, which is a first generation analog mobile communication system, a 30-year history of mobile communication commercialization has become a vital part of our everyday lives while going with many changes in society. In particular, for several years, a mobile communication network has rapidly developed while making various changes to society at large.
In case of a third generation communication system, popularization of the third generation communication system considerably fell into arrears due to unbalanced development between a mobile communication and a mobile computing. Unlike the third generation communication system, a fourth generation communication has rapidly developed based on computing environment, which has changed from such a personal computer as a desktop, a notebook and the like to such a personal information device as a smartphone, a tablet and the like. In particular, since a recent development of cloud computing environment requires a systematic combination of a higher level between communication and computing, a trend of the recent computing environment will be accelerated further.
Recently, ITU has approved LTE-Advanced and WIMAX-Advanced as a fourth generation IMT-Advanced standard. Hence, a discussion on a fifth generation mobile communication system began in earnest. ITU-R WP5D in charge of IMT, which is an international mobile communication standard, is conducting many activities to attract attention for the fifth generation communication including a Workshop in a regional unit, which is held in 2011 in the name of “IMT for the Next Decade”, and the like. Besides, WWRF has published a first report by performing a study on system requisites and vision of a NG-Wireless system.
As communication service environment has considerably changed in past 10 years, it may be very difficult to anticipate changes for the upcoming 10 years. Currently, such communication standard organizations as ITU-R, WWRF and the like consider following elements of changes for the 5G communication environment targeting in a year of 2020.                Development of a multimedia service centering on a high-definition video service        Provision of a differentiated UX (User eXperience) via a personalized service: provision of a service suitable for a personal preference, a situation, an equipment and the like        Change from device-centered communication environment to user-centered communication environment: As a user possesses a plurality of communication devices, provision of a user-centered service is required. Such services as sharing content, provision of seamless mobility between heterogeneous network, security, and the like is provided.        Expanding an M2M service: According to traffic increase due to the increase of M2M devices, provision of a new M2M based service is required.        Expanding the base of mobile cloud computing environment: a mobile cloud computing is provided via provision of communication environment of less latency and high performance, which are enabled in a manner that all communication environments are combined with a network.        
According to the aforementioned changes of the service environment, core requirements of the 5G communication system in the following are discussing.
1. Bandwidth/Transfer Rate Increase
Various analysis reports have anticipated that mobile device and traffic will increase for the upcoming 10 years with a size bigger than before. In case of the mobile device, although increase of population is not big, it is anticipated that a low-end user equipment is replaced with such an internet user equipment as a smartphone, a tablet and the like and the number of connected user equipment such as an M2M will rapidly increase.
According to a report of Cisco, the amount of total mobile traffic has increased 6 times more for a period from 2008 to 2010 and the report anticipates that the amount of total mobile traffic will increase 26 times more until the year of 2015 and will be 6.3 EB (Exabyte, 1018=260 bytes) monthly [3]. UMTS Forum anticipates that the amount of total mobile traffic will be 127 EB in the year of 2020, which corresponds to 33 times of 3.8 EB in the year of 2010, in a manner of referring to material of IDATE. During the period, the number of mobile terminal is anticipated to be increased to 10 billion from 5 billion.
From the aforementioned trend, it is able to know that the first target of the fifth generation mobile communication system is to increase transfer rate. There are various methods to increase the transfer rate. A first available method may correspond to a method of discovering an additional bandwidth currently not in use and using the additional bandwidth.
A current mobile communication system mainly uses a band less than 3 GHz. Yet, due to a limitation of a bandwidth, a study on a higher band is progressing. In particular, a study on securing system performance according to a modified frequency characteristic in a high frequency band of 2˜6 GHz band is progressing as well.
2. Provision of Uniform Service Quality
In terms of a transmission speed, a 4G communication system has shown much improvement such as implementation of maximum 1 Gbps transmission speed and the like. Yet, a unbalance problem of service quality including a difference more than 30 times between spectrum efficiency in a cell boundary and an average spectrum efficiency of a cell and the like is serious. Due to a motto “provision of a preferred service at anytime and anywhere” required by a 5G communication system, improvement for the unbalanced service quality may become a very important requirement.
In relation to this, although a study on performance enhancement for a cell boundary region is progressing via such technologies as WIFI offloading in a 4G system, a method of adding an assistive cell such as Femto BS and the like, eICIC (Enhanced Inter-cell Interference Coordination), COMP (Coordinated Multi-Point) and the like, it is necessary to provide a uniform service of a higher level.
3. User-Centered Systematic Interworking
In a 4G system, interworking between devices is implemented based on an operation of a user or a determined policy. Yet, in a next generation communication system, technologies used for providing a service of an identical level in various communication environments including systematic interworking between many devices are required together with diversification of a user device. Technologies enabling such technologies as a cellular and a wireless LAN to be systematically interworked with each other without a complex process and technologies providing an optimized service in a best environment via seamless provision of all services while minimizing a human involvement are required.
In order to satisfy the aforementioned requirements, various studies are progressing. In particular, various studies on a communication network field are also progressing.
As a user equipment moves, a handover is necessary in a C-RAN system as well. However, a handover procedure in C-RAN based environment has not been studied yet. It is necessary to newly define a handover procedure in the C-RAN environment corresponding to a new communication system.